Entropically Driven Ring-Opening Polymerization of Strainless

Review pubs.acs.org/CR

Entropically Driven Ring-Opening Polymerization of Strainless Organic Macrocycles Philip Hodge* Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom 5. Some Potential Applications of ED-ROPs 5.1. Greener Method of Polymer Synthesis 5.2. Recycling 5.3. High-Throughput Syntheses 5.4. Synthesis of Polymers with High Molecular Weights 5.5. Reactive Molding and Composites 6. Summary and Future Work Author Information Corresponding Author Notes Biography Acknowledgments References

CONTENTS 1. Introduction 2. Main Features of Entropically Driven Ring-Opening Polymerizations 2.1. Principles of ED-ROP 2.2. Some Aspects of the Mechanism of ED-ROPs 2.3. Comparisons with Classical Step-Growth Polymerizations 2.4. Summary of the Main Features of ED-ROPs 2.5. Closely Related Reaction Systems Involving RCE 3. Sources of Macrocyclic Oligomers (MCOs) 3.1. Some General Points 3.2. Characterization of MCOs 3.3. Synthesis of MCOs Using High-Dilution Methods 3.4. Synthesis of MCOs by Distillation from Reaction Mixtures 3.5. Synthesis of MCOs Using Polymer-Supported Reactions 3.6. Synthesis of MCOs by Cyclodepolymerization in Solution 3.7. Extraction of MCOs from the Products of Condensation Polymerizations 4. Entropically Driven Ring-Opening Polymerizations 4.1. Some General Points about ED-ROPs 4.2. Synthesis of Polyesters by ED-ROPs 4.2.1. MCOs Containing Aliphatic Ester Groups 4.2.2. MCOs Containing Aromatic Ester Groups 4.3. Synthesis of Polycarbonates by ED-ROPs 4.4. Synthesis of Polyamides and Polyurethanes by ED-ROPs 4.5. Synthesis of Polyacetals by ED-ROPs 4.6. Synthesis of Olefin-Containing Polymers by ED-ROP 4.7. Synthesis of High-Performance Aromatic Polymers by ED-ROPs Proceeding via Nucleophilic Aromatic Substitution Reactions 4.8. Synthesis of High-Performance Aromatic Polymers by ED-ROPs Proceeding via Free Radical Reactions © 2014 American Chemical Society

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1. INTRODUCTION In recent years great progress has been made in developing both vinyl polymerization1 and the ring-opening polymerization (ROP) of strained cyclic compounds.2 This has resulted in the availability of a broad range of “living” methods for both types of polymerization, methods that in many cases afford products of predefined molecular weights, narrow dispersities,3 and often control of the end groups.1,2 In contrast, until recently relatively little effort has been devoted to developing further the third major type of polymer synthesis, that is, condensation polymerization.4 This is despite the fact that many condensation polymers such as poly(ethylene terephthalate) (PET) (1), poly(butylene terephthalate) (PBT) (2), poly(bisphenol A carbonate) (3), Nylon-66 (4), Nomex (5), and poly(ether ether ketone) (PEEK) (6) are of major commercial importance; see Chart 1.5 Moreover, all the three major types of natural polymers are condensation polymers. In addition, many condensation polymers, in contrast to vinyl polymers, have the potential to be recycled back to their starting materials, or closely related compounds, relatively easily. They are also potentially biodegradable in, for example, the human body or the soil. Finally, some condensation polymers can be synthesized using green biological methods.6 The present article is concerned with entropically driven ROPs (ED-ROPs), a relatively new approach to the synthesis of condensation polymers. It involves the ring-opening polymerization of strainless macrocycles and so has wide applicability. The subject has been reviewed before,7 but it is an active field; it continues to grow and is now beginning to find applications. Unlike several of the previous reviews, the present one seeks to cover this method of polymerization comprehensively.

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Received: April 25, 2013 Published: January 6, 2014

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dx.doi.org/10.1021/cr400222p | Chem. Rev. 2014, 114, 2278−2312

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MCOs, at high concentration and establishing the RCE results in the synthesis of the corresponding polymer in high yield. Generally about 2% of MCOs remain. However, since in most cases these are much more soluble than the corresponding polymer, the polymeric product can be isolated simply by precipitating the final equilibrated mixture into an appropriate nonsolvent for the polymer. It should be noted that full equilibration may not always be achieved during the reaction period, i.e., the reverse reaction in Scheme 1 may be slow. This might be the case, for example, with certain enzyme-catalyzed reactions where the polymer has difficulty binding to the active site of the enzyme. In such cases polymer is nevertheless formed in high yield but, in the limit, by a kinetically controlled process. The amounts of each oligomer present are then predicted to decrease proportionally to n−1.5.10,16,17 The principles of ED-ROPs outlined above are quite general, and polymerizations can be achieved using many types of reaction. Examples include transesterification, transamidation, transacetalization, olefin metathesis, and nucleophilic aromatic substitution. Indeed, it also applies when the bonds linking the repeat units are hydrogen bonds.18

Chart 1. Some Commercially Important Condensation Polymers

It covers the literature from the earliest work on the topic up to mid-2013.

2. MAIN FEATURES OF ENTROPICALLY DRIVEN RING-OPENING POLYMERIZATIONS 2.1. Principles of ED-ROP

In most cases ED-ROPs exploit classical ring−chain equilibria (RCE); see Scheme 1.8−12 These are the equilibria that exist, in

2.2. Some Aspects of the Mechanism of ED-ROPs

It is of interest to first consider the thermodynamics of ED-ROPs, as the dominant factors in these reactions are significantly different from those of the more common ROPs of strained cyclics such as epoxides and strained lactones.19 ED-ROPs essentially just involve the shuffling of the linkages between the repeat units. Because the vast majority of the MCOs used as starting materials for ED-ROPs are sufficiently large as to be strainless, or virtually so, ΔH for the polymerization is zero or very close to zero. Macrocyclic unbranched alkanes are essentially strainless if the ring size is >14 ring atoms.20,21 With other macrocycles the size above which the ring becomes strainless depends to some extent on the atoms in the ring and on the substituents on the ring. For example, replacing a ring methylene by an oxygen atom decreases strain (Oxygen Atom Effect),21 whereas introducing substituents that result in transannular interactions increases strain. Since ΔH is zero, or very close to zero, and ΔG = ΔH − TΔS, it is evident that the position of the RCE must be determined by entropic considerations, i.e., −ΔG = TΔS. Two types of entropy are relevant here: translational entropy and conformational entropy. The translational entropy of an MCO and the polymer are both greatest at high dilution. Both decrease as the concentration increases. The conformational entropy is essentially independent of concentration, and it increases significantly when the MCOs are converted into polymer.19 A very simple way to visualize this effect is to consider a random polymer coil. When the backbone bond angles are limited to 109.5°, the average end-to-end distance, r, is predicted by the equation:

Scheme 1. Ring−Chain Equilibria (RCE)a

a

ED-ROP = entropically driven ring-opening polymerization. CDP = cyclodepolymerization.

the presence of a suitable catalyst, between a condensation polymer and the corresponding family of homologous macrocyclic oligomers (MCOs). RCE have been of considerable theoretical interest for many years.8−12 They can be regarded as a boundary point between organic chemistry (the MCOs) and polymers. Most of the MCOs present in RCE have ring sizes in the range from ca. 14 ring atoms to more than 100 ring atoms; see, for example, the mass spectra shown in refs 13−15. One topic that has been studied extensively is the proportions of the MCOs present in a family of MCOs. If the MCOs are strainless and full equilibration has occurred, based on the theory of Jacobson and Stockmayer,8 the amounts of each oligomer present are predicted to decrease proportionally to n−2.5 where n is the number of repeat units present.10−12 A valuable feature of RCE is that the position of the equilibria depends greatly on the concentration. Thus, the rate of the forward reaction (ED-ROP) in Scheme 1 is proportional to the product of two concentrations, whereas the rate of the reverse reaction (CDP) is only proportional to one concentration. Typically at concentrations 98% polymer and 30 000, the reactions forming the linkages between the repeat units must proceed in >99.7% yield. In the present case the net result of these various requirements is that no reaction solvent is used and the last traces of excess glycol are removed by stirring the viscous reaction mixture at relatively high temperatures, often under vacuum. It is also crucial that equimolar amounts of diol and diacid are incorporated. In the present example this is achieved simply by distilling out the excess diol until the precise proportions are reached: no more can then be distilled out. In most reaction systems that give condensation polymers and use two monomers, however, the precise ratio is achieved by using very pure monomers and weighing them out with high accuracy, often to 0.1% or better. This can be difficult to achieve on a small, say 100 mg, scale. The dispersity3 of the polymeric product is expected to be 2.0. In contrast to the classical synthesis discussed above, EDROP uses two distinct reactions. In the first reaction, one, or a mixture of an homologous family, of MCO(s) 8 is prepared. In this process all the “end groups” from the monomer units are 2280


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disturbed, the equilibrium will re-establish, and so the system can respond to changes in the prevailing conditions. For example, a dynamic dilute solution of MCOs can be stored indefinitely, but if the solvent evaporates and the concentration rises, polymer will be formed. An example of this, involving olefin metathesis, has been described.26

removed, i.e., in the present case all the methanol is removed. The MCOs 8 that are obtained contain all the repeat units required to build the polymer and in precisely the correct stoichiometric proportions. In the second reaction, the MCOs 8 are ring-opened. There is no need to eliminate any end groups in this second reaction. Because, however, linear polymers have end groups, a few are needed. Generally they are derived from the catalyst or any other appropriate species present, for example, water or traces of linear oligomers or polymer. The ring-opening step can easily be carried out on a small scale. Provided that in the ring-opening reaction the RCE is fully established, the dispersity3 of the polymeric product is again expected to be 2.0.

2.5. Closely Related Reaction Systems Involving RCE

Several interesting reaction systems have been described that also involve RCE, and such systems are clearly very closely related to ED-ROPs. As before, the concentrations under which the reactions are carried out are crucial: dilute reaction conditions result in MCOs, and concentrated conditions result in polymers. It should be noted that interconversions between MCOs will generally proceed via linear species, even if only in relatively small amounts, with, if necessary, the catalyst being the source of the end groups. The most obvious of these related reaction systems is simply the reverse of ED-ROP. In this a dilute solution of a polymer is treated under conditions that achieve RCE. Because of the dilution, the position of the equilibrium shifts to give the MCOs in high yield; see Scheme 1. This reaction, cyclodepolymerization (CDP) or ring-closing depolymerization (RCD), can actually be used to prepare MCOs and so is discussed more fully in section 3.6. A second interesting system involves establishing the RCE starting with a dilute solution of a single MCO. No polymer is formed because of the dilution. Instead an homologous family of MCOs is generated. For example, a cyclic monomer is transformed into a family of homologous MCOs. The product can be viewed as a library of homologous macrocycles with a range of ring sizes. This type of reaction has been termed ringexpanding oligomerization (REO), and such reactions have been of interest for a number of years.14,27 One example is the treatment of crown ether analogue 9 in 1,2-dichlorobenzene at 140 °C for 24 h with a catalytic amount of di(n-butyl)tin oxide to give library 10 consisting of 2% of the 9-membered ring, 81% of the 18-membered ring, 11% of the 27-membered ring, 3% of the 36-membered ring, and 3% of larger rings; see Scheme 3.28

2.4. Summary of the Main Features of ED-ROPs

While bearing in mind the above discussion, the main features of ED-ROPs can now be summarized. They are as follows. (1) ED-ROP permits the polymerization of large strainless rings, not just small strained rings. This greatly increases the scope of ROPs as the large rings can contain one or more substantial moieties. (2) As the reactions just involve a shuffling of the linkages between repeat units, no small molecules, and hence no volatiles, are evolved during the polymerization. This helps to make ED-ROPs environmentally friendly. (3) As most, if not all, of the macrocycles in an homologous family of MCOs are strainless, little or no heat is evolved in ED-ROPs. (4) The MCOs have no end groups, but obviously end groups are required to produce linear polymers; see Scheme 1. Accordingly, the final molecular weight of the polymer depends, at least in part, on the number of end groups present in the system. Typically the catalyst used to bring about the equilibration serves as the source of end groups. Minimizing the number of end groups brings the possibility of obtaining polymers with unusually high molecular weights. (5) In cases where the MCOs contain two types of repeat unit, for example, diacid and diol, each MCO automatically contains a perfect stoichiometric balance of the two types of repeat unit. Accordingly, stoichiometric balance is not an issue in ED-ROPs, as it can be with classical condensation polymerizations, and this allows very small-scale polymer syntheses, say 100 mg, to be carried out quite readily. (6) As it is necessary to have high concentrations of the MCO(s), it is often convenient to carry out polymerizations without solvent and without stirring. (7) If full equilibration is to be achieved, there must be some mobility in the system; and this even needs to be the case when the mixture consists of >95% polymer. Hence with a neat reaction mixture, even if only relatively briefly, the polymerization temperature should exceed the Tg of the product and most probably also the Tm. (8) Provided the RCE is fully established, the dispersity3 of the product is expected to be 2.0, i.e., the same as that expected for all classical step-growth polymerizations. (9) If two monomers are used, the final copolymer should, with sufficient reaction time, have the thermodynamically most probable distribution of repeat units. Thus, if the linkages in two monomers are in a similar structural environment, the final product is expected to be a random copolymer. (10) If the prevailing conditions are such that the MCOs and the polymer are interconverting, the system can be described as being dynamic. With such a system, if the position of the RCE is

Scheme 3. Some Syntheses of MCO Libraries by REO of Strainless Macrocycles

Similarly, treatment of cyclic dimer 11 in chlorobenzene at 130 °C for 24 h with a catalytic amount of distannoxane 12 gave the family of MCOs 13 with the macrocycles larger than the dimer present in 41% yield;14 see Scheme 3. The REO of ε-caprolactone has also been reported.29 REOs are useful as, for example, they can be used to transform a single macrocycle of high melting point and/or low solubility into a mixture that has 2281


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combinatorial library of MCOs will be obtained. For example, reaction of a dilute solution of macrocycles 16−18 in chlorobenzene with 3 mol % of di(n-butyl)tin oxide at 183 °C gave library 19, which contained six 18-membered rings (75% by wt) and ten 27-membered rings (15% by wt); see Scheme 5.32 A similar library was obtained by carrying out a CDP starting with a mixture of the three corresponding polymers.32 Similarly cyclics 16−18 plus the analogue with X = NCH3 gave a library with ten 18-membered rings and twenty 27-membered rings.32 If the above libraries of MCOs are isolated, they can be considered as static combinatorial libraries (SCLs): if the MCOs continue to equilibrate, they are dynamic combinatorial libraries (DCLs). Studying the way the position of equilibrium of a DCL shifts in the presence of a potential “guest” of one or more MCOs is currently being used successfully to identify significant host−guest combinations.33 Interestingly, if the equilibration of library 10, mentioned above, is carried out in the presence of the potassium salt form of Amberlyst 15 (a macroporous polystyrene network bearing sulfonic acid residues), the percentage of the cyclic dimer (18-membered ring) increases from 81% to 94%,28 presumably because this macrocycle interacts favorably with the potassium cation.

a lower melting point and/or better solublility. This is helpful for some ED-ROPs; see section 3.1. REO of cyclic polymers is also a possibility, but cyclic polymers have only recently become readily available;30 at the time of this writing, no examples of the REO of cyclic polymers have been reported. If the type of experiment discussed in the previous paragraph is carried out with a highly strained macrocycle, at equilibrium very little of the starting material may remain. Thus, reaction of the strained macrocycle 14 with cesium fluoride in N,Ndimethylacetamide (DMAc) at 150 °C gave the homologous series of macrocycles 15; see Scheme 4.31 Scheme 4. Synthesis of an MCO Library by REO of a Strained Macrocycle31

3. SOURCES OF MACROCYCLIC OLIGOMERS (MCOS) 3.1. Some General Points

Clearly for ED-ROPs to be useful the required MCO(s) must be readily available. Some monomers, such as pentadecanolide (20), hexadecanolide (21), musk compound 22, and α-, β-, and γ-cyclodextrins (23: x = 6, 7, and 8, respectively), are commercially available; see Chart 2. Others are readily prepared from commercial materials, for example, the O-permethyl ether of β-cyclodextrin.34 In most cases, however, the MCOs need to be specially prepared. Some of the methods available are discussed in this section. Since macrocycles have been prepared in many ways and for many purposes other than for ED-ROPs, the coverage here cannot be comprehensive. Only some of the more important examples relevant to ED-ROPs are considered.

If the experiments discussed in the two preceding paragraphs are carried out using mixtures of different families of MCOs, a

Scheme 5. Synthesis of a Macrocycle Library from a Mixture of MCOs32

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N−H signals.40 Other examples of the use of 1H NMR spectroscopy to characterize MCOs are to be found in refs 41−43.

Chart 2. Some Commercially Available MCOs

Size-exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), is a powerful tool to characterize MCOs because stationary phases are commercially available that can resolve many MCO families with from ca. 14 up to ca. 80−100-membered rings; see Figure 1.15,21,37,38,40,41,44−46 Plots of log(Degree of Polymerization) versus Elution Volume are generally linear,14,15,40,44−47 and, because a given MCO is hydrodynamically smaller than the corresponding linear species, the plot for a family of MCOs is different from the plot for the corresponding linear oligomers. Such plots can be of considerable assistance in making peak assignments. High-pressure liquid chromatography (HPLC) is also powerful for analyzing quantitatively mixtures of MCOs. See refs 36, 37, 40, 42, 43, and 48 for examples. Mass spectrometry is valuable because a family of MCOs gives a series of peaks different from the corresponding linear oligomers. Usually matrix-assisted laser desorption ionization time-of-flight (MALDI ToF) mass spectrometry has been the preferred type of spectrometry; see Figure 2 and refs 14−16, 37, 42, 44 and 45 for examples of its use. Occasionally fast atom bombardment (FAB) mass spectrometry has been used; see refs 36, 44, 41, and 48 for examples. Finally, an isolated pure solid MCO can have its structure, and hence cyclic nature, demonstrated by X-ray crystal structure determination and many such studies have been reported; see, for example, Figure 3. Other examples include a macrocyclic carbonate, 49 macrocyclic carboxylate esters,14,45,50−53 a macrocyclic amide,54 and macrocycles related to high-performance aromatic polymers.13,15,55−60 An X-ray crystal structure for one of the latter is for a macrocycle containing 92 ring atoms.31

Full details of the source of particular MCOs will be found in the papers discussed below that consider their ED-ROPs. It is important to note first that many methods for obtaining MCOs actually initially afford not just one MCO but a family of homologous MCOs. Fortunately such mixtures are perfectly acceptable as starting materials for ED-ROPs because the repeat unit is the same for all members of an homologous family and so they all polymerize to the same polymer. Indeed, in many cases using a mixture of MCOs is actually more beneficial than using one pure compound because the mixtures generally have lower melting points, have lower melt viscosities, and are more soluble. As an example, consider PET (1). This polymer melts at ∼270 °C,35,36 and it begins to decompose at ∼300 °C.36 The pure cyclic trimer 8: x = 3 actually melts at 321 °C,35,36 i.e., higher than both these temperatures, whereas the mixture of MCOs 8 can, depending on the precise composition, melt at 92% cyclic. It should be noted that the use of di-n-butyltin

interactions are possible on 1% cross-linked polystyrene beads.108,109 There has been considerable interest in using the supported method to prepare ester-containing MCOs. Rothe and Zieger used the same approach as they used to synthesize the amidecontaining MCOs 97 to synthesize lactones 99 and 100.111 The hydroxy acids were loaded on to the functionalized polymer beads with the hydroxyl groups protected as O-tert-butyldimethylsilyl 2291


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There is a limit to the amount of linears that can be present in the products from CDPs because each linear molecule formed requires there to be a source of two end groups and there are generally very few present in the system. This is particularly the case with, for example, reactions involving olefin metathesis because there are unlikely to be adventitious olefins present. The limit on linears can be illustrated by a very simple example. Thus, if a polymer chain of degree of polymerization (DP) 200 is degraded to oligomers of average DP 3, then 67 oligomers molecules will be formed. However, the only end groups present will be any from the catalyst plus the two that were on the original polymer chain. Neglecting the former, the percentage of linear oligomers is limited to (1/66) × 100 = 1.5%. Polyesters are some of the most readily cyclodepolymerized polymers, and many such reactions have been studied. For example, CDP of polyundecanoate was achieved by heating a 2% w/v solution in chlorobenzene at ca. 133 °C for 8 h with a catalytic amount of di-n-butoxydi-n-butyltin. This gave the family of MCOs 101 in 90% yield.14 Similar reactions gave the families of MCOs 100−102 in excellent yields.14 MCOs 112−115 have been obtained using a lipase to catalyze the depolymerizations.119−121 All the common commercial aromatic polyesters plus several similar ones have been subjected to CDP. The reactions gave the families of terephthalate-containing MCOs 8,38,122,123 29,51,124 31,125 37,46,126 116,45 117,41 and 118127 and the isophthalate-containing MCOs 33,128 35,128 and 39.128 It should be noted that the MCOs obtained by CDP can contain traces of the catalyst, probably linked to some of the end groups, and this can cause the MCOs obtained to repolymerize simply by heating.

oxide as the catalyst is a key part of the method. The use of a catalytic amount of sodium methoxide in place of the di-nbutyltin oxide was unsuccessful because much of the hydroxy acid became detatched. It appears that the di-n-butyltin oxide activates both the hydroxyl group and the ester linking group, and it may also help to bring them together.116 The method provides efficient small-scale syntheses of, for example, MCOs 13, 101−105,116 and 106;117 see Chart 6. One attraction of this Chart 6. MCOs Prepared Using Polymer-Supported Reactions

general method is that polymer-supported (PS) ω-hydroxy acids can be assembled on the beads by solid-phase organic synthesis prior to cyclization. For example, ester-amide 48,40,47 MCOs 107 and 108,74,116 depsipeptide MCOs 109 and 110,118 and 111117 were prepared this way. Accordingly the method has the potential to be used for the preparation of combinatorial libraries of MCOs and, hence, via ED-ROPs, of combinatorial libraries of polymers. 3.6. Synthesis of MCOs by Cyclodepolymerization in Solution

Cyclodepolymerization (CDP) is the conversion of a polymer into MCOs. In section 3.4, the CDP of some polymers was achieved by adding a catalyst that helped to interconvert the polymers and the corresponding MCOs, and then distilling out the more volatile cyclics. The examples included the synthesis of cyclic monomers and/or dimers containing carbonate, ester, anhydride, or acetal linkages. This section is concerned with a more widely applicable type of CDP, i.e., depolymerization of a dilute solution of a condensation polymer. It is based on the RCE discussed above and is the reverse of ED-ROP; see Scheme 1. The MCOs obtained can range in size from monomers to oligomers with >100-membered rings. Given a good supply of the starting polymer, this can be a very useful way of obtaining certain MCOs. In practice CDP typically involves taking a hot dilute solution (typically 85%, and this method can be used to obtain tens of grams, possibly even kilograms, of MCOs.

CDP of polycarbonate 3 in 1,2-dichlorobenzene at ca. 180 °C for 5 days using tetra-n-butylammonium tetraphenylborate (TBATPB) as a catalyst gives MCOs 27 in high yield.102 This CDP can also be achieved in moderate to high yields using cesium hydroxide or potassium hydroxide as the catalyst.129 TBATPB also serves as a catalyst for CDPs that give carbonatecontaining MCOs 81 and 82, but with these aliphatic polymers a reaction time of only 3 days is required.102 CDP of polyamides is generally less straightforward than the CDP of polyesters, partly because reversibly breaking amide linkages is much more difficult than reversibly breaking ester linkages, and partly because polyamides are often not very soluble. Nomex (5) has, however, been successfully subjected to CDP.44 This involved treating 5 in dimethyl sulfoxide, containing lithium chloride to increase amide solubility,75 at 2292


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150 °C for 70 h using the sodium salt of benzanilide as the catalyst. The yield of MCOs 44 was 93%.44 Polyurethanes 119 and 120 have also been successfully cyclodepolymerized by treating suspensions of the polymers in 1,2-dichlorobenzene at ca. 180 °C with a catalytic amount of TBATPB. The products were MCOs 121 and 122.130

3.7. Extraction of MCOs from the Products of Condensation Polymerizations

This approach is obviously similar to the CDPs discussed in the previous section, but in this case no reaction is carried out to increase the content of MCOs. The products of condensation polymerizations generally contain only a small fraction, typically ∼2% w/w, of MCOs. Because in most cases the MCOs are much more soluble than the corresponding polymer, they can be extracted easily, typically using a Soxhlet apparatus. MCOs have been obtained in this way for many years, and the results have been summarized.10,35 More recent examples are the extraction of MCOs 29 from polyester 2,133 MCOs 61 from polyetherketone 128,55 MCOs 129 from poly(1,3-dioxolane),134 MCOs 130 from poly(ethylene isophthalate),135 and co-MCOs from poly(ethylene terephthalate)-poly(ethylene isophthalate) copolymers.135 This general approach is clearly limited to condensation polymers that are available in quantity. Even then it is generally only practical to extract a few grams of MCOs. For some applications, such amounts are, however, quite sufficient.

Numerous olefin-containing polymers have been subjected to CDP,7g,74,75,131,132 usually using Grubbs second-generation catalyst 50. With this catalyst there is, however, a possibility of small amounts of double-bond migration.74 Olefin-containing MCOs prepared in this way are 52−55,74,75 123,131 and 124.132 Interestingly a significantly higher proportion of 123: x = 1 is obtained if the CDP is carried out in the presence of a Li+template.131

In a particularly interesting piece of work, the MCOs 8 (ca. 2% w/w) in a commercial sample of PET (1) were first extracted with boiling dioxane. 136 The MCO-free polymer was then heated at 285 °C, i.e., molten, under vacuum, and the MCO content was then estimated. Over a period of 4 h, the RCE slowly became established with the MCO 8 content eventually returning to ca. 2% w/w. Thus, reactions occurred despite the fact that no catalyst was added.

Many high-performance aromatic polymers have been successfully subjected to CDP. The reactions are usually based on nucleophilic aromatic substitution reactions, and the catalyst is usually either fluoride anion or a phenolate anion. The CDP of polymer 125 (Radel) is typical.15 The reaction was achieved by treating polymer 125 in N,N-dimethylacetamide at 165 °C for 72 h with a catalytic amount of cesium fluoride. The yield of MCOs 68 was ∼90%.15 MCOs 64,81 67,13 and 12655 were prepared similarly. CDP of polymer 127 to give MCOs 76 was achieved by treating the polymer in N,N-dimethylacetamide at ∼150 °C with a catalytic amount of diphenyl ether 4,4-dithiol and potassium carbonate.91

4. ENTROPICALLY DRIVEN RING-OPENING POLYMERIZATIONS This section, the main purpose of this review, presents specific examples of ED-ROPs. In general only starting materials where the MCO, or the main MCO present in a mixture of MCOs, has 14 ring atoms or more are included. It is convenient to group the syntheses according to the linkages that react, although certain types of polymers may of course be obtained in more than one way. For example, an unsaturated polyester may be obtained via transesterifications or via olefin metatheses. It should be noted that the structures of the polymeric products from the two methods are, of course, not necessarily the same. 4.1. Some General Points about ED-ROPs

As noted previously, ED-ROPs need to be carried out using neat MCOs or, failing that, MCOs in solution at high concentration. For a catalyst to be effective, it obviously needs to be in close molecular contact with the MCOs. Contact is clearly limited if the catalyst is a solid that does not melt or dissolve in the reaction mixture. When, however, reaction does begin, in many cases catalytic activity will be created at a minimum of one oligomer or polymer end group, 2293


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Table 1. Aliphatic Polyesters Prepared by ED-ROP Using Metal-Containing Catalysts or Organic Base 133 entry polymer 1 2 3 4 5 6 7 8 9 10 11 12

131 131 131 131 136 138 139 141 143 144 145 146

MCO(s)

sourcea

ring atoms in major MCO

reaction conditionsb

catalystc

temp (°C)

time (h)

yield

mol. wt.d

dispersitye (Đ)

ref.f

20 20 20 20 135 137 13: x = 1 → 10 140 142 9 94 95

com com com com HD distill CDP CDP CDP distill distill distill

16 16 16 16 14 12 24 30 28 18 12 21

neat tol neat neat tol CH2Cl2 film neat neat neat neat tol

130 130 YP 133 134 134 12 DBTO DBTO SnOct SnOct SnOct

100 100 80 100 rt rt 170 200 250 130 130 130

4 4 0.5 96 0.67 1 48 12 2 20 20 144

99 95 100 99 90 88 ca. 100 100 96 85 89 93

41.0:106.6 100.0:240.0 15.0:31.5 23.8:35.7 26:44 53.0:84.8 46.2:81.6 20.0:51.0 14.9:28.3 7.5:12.8 9.0:16.8 11.4:21.7

2.6 2.4 2.1 1.5 1.7 1.6 1.77 2.55 1.9 1.7 1.87 1.9

139 139 140 141 142 108 14 143 24 104 104 104

a

Source of the MCO(s): com = commercial; HD = high-dilution synthesis; distill = distilled from a reaction mixture; chrom = obtained by chromatography of a mixture obtained by CDP. CDP = cyclodepolymerization. bReaction carried out with neat MCO(s) or with MCO(s) in solution, in which case the solvent was as indicated. tol = toluene. cMetal-based catalysts generally used at ca. 2 wt %. YP = yttrium isopropoxide. DBTO = di-n-butyltin oxide. SnOct = tin octanoate. dBy SEC relative to appropriate standards. Molecular weights expressed in 1000s. They are called molecular weights here because that is generally as they were described in the publications listed under “refs” in the final column. eFrom SEC data. See ref 3 for IUPAC’s recent recommendations on dispersity, previously called polydispersity. fReference to the ED-ROP.

because the reaction mixture soon becomes very viscous. Diffusion problems probably then limit the molecular weights. Also, there is a tendency for the polymeric product 131 to crystallize.139 Recently it has been shown that the ED-ROP of pentadecanolactone (20) can also be achieved using an yttrium isopropoxide catalyst (see entry 3)140 or the organic base catalyst 1,5,7-triazabicyclo[4,4,0]dec-5-ene (133) (see entry 4).141 The zinc complex 134 has been shown to be an excellent catalyst for some other ester-containing MCOs; see entries 5 and 6.108,142

and this oligomer or polymer will usually mix well with the MCOs. To obtain linear polymers it is clearly necessary have a source of end groups; see Scheme 1. These may come from the catalyst itself, traces of linear oligomers or similar species present in the reaction mixture, or adventitious compounds, e.g., water. The number of end groups present in the ED-ROP can have a major effect on the molecular weights of the polymeric products, and if very few end groups are present, for example, in an ED-ROMP where no other olefinic compounds other than the catalyst, MCO(s), and polymer are present, very high molecular weights may be obtained. 4.2. Synthesis of Polyesters by ED-ROPs

Many ED-ROPs leading to polyesters have been reported. The syntheses generally proceed by repeated transesterifications. These can be catalyzed by metal-containing catalysts, for example, di-n-butyltin oxide, tin octanoate, distannoxane 12, or titanium tetraisopropoxide, by organic bases, or by lipase enzymes. It is convenient for the following discussion to split the various ED-ROPs into three groups: (i) those involving aliphatic esters of all types; (ii) those involving esters of aromatic acids; and (iii) those involving other esters such as aryl esters and aryl thioesters. 4.2.1. MCOs Containing Aliphatic Ester Groups. The results obtained with aliphatic esters catalyzed by metal-containing species and by an organic base are summarized in Table 1. It is clear that in general yields are high and Mn values are in the range 9 000−100 000 with dispersities in the range 1.5−2.6. The ED-ROP of pentadecanolactone (20) has been studied several times; see, for example, Table 1, entries 1−4, partly because the monomer is readily available and partly because the polymeric product 131 is sometimes considered to be a green alternative to polyethylene.137,138 An excellent catalyst for the polymerization is the aluminum−salen complex 132, especially when used in solution; see entries 1 and 2.139 When the pentadecanolactone (20) is polymerized using this catalyst in toluene solution, polymers with particularly high molecular weights are obtained; see entry 2. When the pentadecanolactone (20) is used neat, the results are less satisfactory

It is interesting to note that the family of MCOs 13 polymerized well as a film; see entry 7.14 Thus, a film of the MCOs plus a catalytic amount of distannoxane 12 was cast on a 2294


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microscope slide. When the film was heated to 170 °C, polymerization occurred and a self-standing polymeric film was formed. Polymers 141 and 143−146 were also prepared satisfactorily from appropriate monomers; see entries 8−12.

close to 100 000. Dispersities are usually close to 2.0. It was shown at an early stage that the reactions proceed well in toluene, but only moderately well in water.143 The substrates polymerized include pentadecanolactone (20) (see entry 8) and substrates containing olefinic groups (see entries 17−19) and thioesters (see entry 21). Ambrettolide epoxide, i.e., the epoxide formed from MCO 157, has also been successfully polymerized.144 The polymerizations of steroidcontaining MCOs (entries 19 and 20) also demonstrate the viability of including major structural features in the MCOs. A mixture of MCOs 13 was separated chromatographically to give pure samples of the cyclic monomer, dimer, trimer, tetramer, and pentamer.145 All these homologues polymerized satisfactorily; see Table 2, entries 2−6. In the case of the pentamer, it demonstrates that 60-membered rings can undergo ED-ROP satisfactorily with a lipase as the catalyst. Bearing in mind the percentages of each cyclic oligomer present in the starting materials and the yields of the final polymers, the EDROP of MCOs 100 (entry 7) demonstrates that macrocycles with 84-membered rings can be polymerized successfully. Similarly the ED-ROP of 162 demonstrates that macrocycles with 38-membered rings can be polymerized successfully; see entry 18. The kinetics of the reactions of a range of lactones with CALB have been studied and compared with those using other enzymes and metal-containing catalysts.150,151 For example, zinc octanoate and Pseudomonas f luorescens were used as catalysts for the ED-ROPs of a range of lactones.150 In general metal octanoates were more effective for lactones with up to 9 ring atoms, while the CALB was more effective for lactones having 16-membered rings and larger.150,151

A particularly interesting feature of MCOs containing aliphatic ester groups is that, where enzyme selectivity allows (the presence of alkyl-CO2-alkyl group is required), the EDROP tranesterifications can be catalyzed by lipases, most commonly polymer-supported Candida antarctica lipase B (CALB), marketed commercially as Novozyme 435.6 The environmental attractions of using enzymes as catalysts has led to many such studies. The main results are summarized in Table 2. Many of the relevant formulas are shown in Chart 7. The subject has been reviewed.6 It is evident that a wide range of aliphatic polyesters can be prepared using lipases. Molecular weights are often high, and several polymers have been prepared with Mw, for example,

Table 2. Aliphatic Polyesters Prepared by ED-ROP Catalyzed by Polymer-Supported Candida antarctica Lipase B entry polymer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

139 139 139 139 139 139 147 131 148 149 150 152 154 156 158 160 161 163 164 166 168

MCO(s) 13: x 13: x 13: x 13: x 13: x 13: x 100 20 21 114 113 151 153 155 157 f 159 g 54 162 56 165 167

= = = = = =

1 → 10 1 2 3 4 5

sourcea


reaction conditionsb

temp (°C)

time (h)

CDP com chrom chrom chrom chrom PS com com CDP CDP HD gift gift com com HD HD HD HD HD

24 12 24 36 48 60 26 16 17 12 10 18 16 17 17 16 28 38 35 29 18

tol tol tol tol tol tol tol tol tol tol tol neat neat neat tol tol tol tol tol tol neat

70 70 70 70 70 70 70 60 60 70 60 120 60 75 60 60 70 70 70 70 120

20 20 20 20 20 20 20 4 4 6 6 48 6 24 4 4 20 20 20 20 48

yield 93 93 99 98 97 98 97 ca. ca. 75 86 94 97 96 ca. ca. 95 91 88 92 90

100 100

100 100

mol. wt.c

dispersityd (Đ)

ref e

63.4:120.5 49.0:97.7 59.4:93.4 16.8:28.6 16.6:31.5 13.0:26.2 55.0:110.0 45.8:91.1 18.5:35.9 12.4:21.0 32.5:52.0 104:219 4.1:9.8 4.1:9.0 24.2:44.6 24.0:44.6 23.5:44.6 17.9:32.2 18.2:32.6 25.4:49.5 50:115

1.90 1.99 1.57 1.70 1.90 2.01 2.00 1.99 1.94 1.7 1.6 2.1 2.4 2.2 1.92 1.86 1.90 1.8 1.79 1.95 2.3

145 145 145 145 145 145 145 146 146, 147 120 120 148 149 149 146 146 146 146 145 145 148

a

Source of the starting MCO(s): CDP = cyclodepolymerization; com = commercial; chrom = obtained by chromatography of a mixture obtained by CDP; PS = polymer-supported synthesis; HD = high-dilution synthesis. bReaction carried out with neat MCO(s) or with MCO(s) in solution in toluene (= tol). cBy SEC relative to appropriate standards. Molecular weights expressed in 1000s. They are called molecular weights here because that is generally as they were described in the publications listed under refs in the final column. dFrom SEC data. See ref 3 for IUPAC’s recent recommendations on dispersity, previously called polydispersity. eReference to the ED-ROP. fCompound 157 = Ambrettolide. gCompound 159 = Globalide. 2295


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but unfortunately, to date, no enzymes have been found that react well with aromatic esters. Because of the commercial importance of aromatic polyesters, many ED-ROPs that produce them have been studied; see Table 3. Goodman and Nesbitt described in 1960 the extraction of a mixture of MCOs 8 (1.3−1.7 wt %) from commercial PET (1) chip or fiber and then the careful isolation of pure samples of the cyclic trimer, the cyclic tetramer, and the cyclic pentamer from the mixture.152−154 All three of these MCOs underwent ED-ROP when treated with a catalytic amount of antimony oxide at 275−310 °C, with the choice of reaction temperature depending on the melting point of the MCO; see Table 3, entries 1−3.152−154 With this catalyst no reaction occurred unless a source of end groups was available, often simply adventitious water.152−154 They also showed that PET (1) itself could be used as a reaction solvent for the ED-ROPs.154 More recently, Nagahata and co-workers collected the cyclic dimer 8: x = 2 that had volatilized onto the surrounds of processing equipment at a PET (1) film-producing plant.155 Using differential scanning calorimetry (DSC) pans as reaction vessels, the dimer was heated alone and with various catalysts. ED-ROP occurred in all cases to give PET (1) (entries 4 and 5), with the catalyzed reactions generally giving cleaner products and higher molecular weights.155 Both Burch’s and MacKnight’s research groups reported in 2000 that, for ED-ROPs leading to polymer 1, a more satisfactory starting material than individual macrocyclic oligomers is the mixture of MCOs 8.36,116 This is because the mixtures generally have lower melting points; see section 3.1. They confirmed that PET (1) itself could be used as a reaction solvent.36,122 MacKnight’s research group also showed that antimony oxide and bismuth oxide, again in the presence of a source of end groups, were satisfactory catalysts for achieving ED-ROP with the mixture 8; see entries 6 and 7.122,123 ED-ROPs of MCOs 8 were also investigated using a DSC instrument and stannoxane 12 as the catalyst; see entry 8.46 With a temperature of 280 °C and a reaction time of 1 h, PET (1) was obtained with an inherent viscosity 82% that of commercial material. However, after the product was washed with chloroform to remove oligomers, the residue (95% of the original weight) had an inherent viscosity 9% higher than that of commercial material.46 PET (1) has been prepared on a 100 mg scale by treating MCOs 8 with a catalytic amount of di-n-butyltin oxide at 300 °C; see entry 9.156 Finally, a film of MCOs 8 plus distannoxane catalyst 12 was cast from chloroform onto a microscope slide, and the film was heated at 265 °C under nitrogen for 1 h; see entry 10.46 This afforded a self-standing film of polymer 1. Poly(trimethylene terephthalate) (PTT) (169) has been prepared on a small scale by the ED-ROP of the mixture of MCOs 11645 and of the isolated dimer (116: m = 2); see entries 11 and 12.45 A range of copolymers were prepared from mixtures of MCOs 116 plus either 8 or 14.45 In 1998 Brunelle’s group at General Electric described in some detail the ED-ROPs of MCOs 29 and of the cyclic cooligomers derived from a 5:95 mixture of MCOs 8 and 29. The co-oligomers were prepared with a view to ultimately using them in ED-ROPs in the preparation of composites.53 The cooligomers have the advantage of a lower melting point than either of MCOs 8 or 29, and also the product is slower to crystallize. Polymerizations were carried out at 190 °C by adding a solution of the catalyst in a minimum volume of solvent to the stirred molten MCOs under a nitrogen atmosphere. The reaction time was only 20 min. Selected results are summarized in Table 3, entries 13 and 14. For similar reasons

Chart 7. Some Macrocycles That Can Be Polymerized by Candida antarctica and Some of the Polymers That Are Produced

4.2.2. MCOs Containing Aromatic Ester Groups. The synthesis of aromatic polyesters by ED-ROP is usually less straightforward than the synthesis of aliphatic esters because the MCOs used for the former generally have higher melting points and lower solubilities than the latter, as do the polymeric products. Moreover the products can crystallize rapidly, thus limiting diffusion of unreacted MCOs and other reactants in the reaction mixture, especially toward the end of the polymerization. Many metal-containing catalysts are very effective, 2296


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Table 3. Aromatic Polyesters Prepared by ED-ROP reaction conditionsc entry polymer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

1 1 1 1 1 1 1 1 1 1 169 169 2 2k 2 2 2 170 171 172 172 173 176 176 176 177 178 179

MCO(s)

sourcea

ring atoms in major MCOb

catalystc

temp (°C)

time

yield (%)

mol. wt.d (Mn:Mw) or viscositye

8: x = 3 8: x = 4 8: x = 5 8: x = 2 8: x = 2 8 8 8 8 8j 116 116: x = 2 29 29k 29 29 29 31 118 174 175 35 37 37 37: x = 3 38 42 43: x = 3

extr extr extr extr extr CDP CDP CDP CDP CDP CDP CDP HD HD com com com CDP CDP subl CDP CDP HD CDP CDP HD HD HD

30 40 50 20 20 30 30 30 30 30 22 22 24 24 24 24 24 28 42 18 18 26 36 36 36 28 38 30

AO AO AO none DBTC AO BO 12 DBTO 12j DBTO DBTO TOT TOT BTCDH TEHT 12 AO AO AO DBTO DBTO TTP DBTO DBTO DBTO TBATPB none

306 275 275 300 250 293 293 280 300 265 300 300 190 190 205 200 195 200 290 292 290 250 295−300 350 350 275 310 260

15 min 15 min 15 min 2h 4h 10 min 10 min 1h 2h 1h 2h 2h 20 min 20 min ca. 30 min ca. 30 min ca. 30 min 20 min 20 min 2h 8h 2h 25 min 10 min 10 min 15 min 40 min ca. 20 min

high high high 93 96 high high high high high high high 96 98 97 95 95 92 97 93 76 98 high high high high high 83

[η] 0.67h

9.4:16.0 --:46.0 25.3:-32.4:-ηinh 0.55i 15.7:34.5 ηinh 0.40i Mv 22.5 Mv 30.3 --:104.1 --:121.1 50.1:130.3 54.7:136.8 42.2:118.2 18.0:44.3 17.0:48.1 [η] 0.50m 8.2:-21.3:45.4 ηinh 0.24n ηinh 0.35n ηinh 0.48n ηinh 0.28l 51.0:96.0 --:44.0

dispersity f (Đ)

1.7

2.2

2.6 2.5 2.8 2.1 2.8

2.1

1.9

ref g 152−154 152−154 152−154 151 151 118, 122, 123 118, 122, 123 46 156 46 45 45 53 53 157 157 123, 157 125, 158 127 159 156, 160 124 69 46 46 69 70 71

a Source of MCO(s): extr = by extraction of solid polymer; CDP = cyclodepolymerization; HD = high-dilution synthesis; com = commercial; sub = sublimed from polymerization reaction. bNumber of ring atoms in the most abundant MCO in the starting material. cAll ED-ROPs were carried out using neat MCOs. Catalysts generally used at 280 °C has been carried out using a DSC instrument.188

Scheme 13. Some Well-Known Reactions of Thioethers

especially if the residues include metal ions. This problem might be minimized if the ED-ROPs proceed via free radical reactions. The three reactions shown in Scheme 13 have been known for many years. Reaction 1 is the reaction of diphenyl sulfide with elemental sulfur to give diphenyl disulfide.181 Reaction 2 demonstrates the exchange of sulfur atoms between diphenyl sulfide and elemental sulfur,182 whereas reaction 3 is the disproportionation of phenyl biphenyl sulfide to give diphenyl sulfide and bis(biphenyl) sulfide.183 All these reactions are expected to be fully reversible. Knowledge of these reactions prompted Hay’s group in Montreal to develop the ED-ROPs of macrocycles that include aryl sulfur linkages. They soon found that such MCOs undergo ED-ROPs when they are treated with catalytic amounts of elemental sulfur or an aromatic disulfide at elevated temperatures.184 Representative examples are summarized in Table 9. Others examples are presented in the text. The reactions are believed to proceed by free radical substitutions. In these reactions, aryl-S radicals (thiyl radicals) attack at aryl-S centers, and in the process new thiyl radicals are generated; see Scheme 14.184,185 Scheme 14. Main Type of Reaction Occurring in Free Radical ED-ROPs

Similar ED-ROPs afford polymers 254,185 256,88 and 257;88 see Table 9, entries 4−6, respectively, and 251.189 MCOs containing disulfide or polysulfide linkages, for example, 76 and 77, have also been polymerized.92,93,190 More recently, further ED-ROPs have been studied that are believed to be initiated simply by the themolysis of an aryl-S bond, in this case activated by being para to an electron-

Table 9. High-Performance Aromatic Polymers Prepared by ED-ROP Using Free Radical Reactionsa entry polymer 1 2 3 4 5 6 7 8

251 252 253 254 256 257 263 263

MCO(s)


catalystb

temp (°C)

248 249 250 255 72 73 261 262

25 38 40 28 40 40 30 40

247 247 S S 247 247 none none

340 350 350e 300 285 285 up to 480 f up to 480 f

time (h)

conversion (%)

mol. wt. (Mn:Mw) or viscosityc

3 0.5 0.5 0.5 ca. 0.07 ca. 0.07

high high >90 >90 >90 >90 high high

d d 28.2:77.0 31.0:73.0 d d ηinh ca. 1g ηinh ca. 1h

dispersityc (Đ)

2.7 3.4

ref 184, 186 184, 187 184, 185 189 88 88 191, 192 191, 192

a

Unless indicated otherwise, ED-ROPs were carried out using neat MCOs. All MCOs prepared by high-dilution syntheses. bCompound 247 is 2,2′dithiobis(benzothiazole) (DTB). cMolecular weights and dispersities estimated by SEC. Molecular weights expressed in 1000s. Units of viscosity are dL/g. dSEC not possible because the product was not soluble in the SEC solvent. However, the thermal properties determined by DSC match literature values for the polymer. eSolvent m-terphenyl. fReaction carried out in a DSC instrument. gConcentration 0.5% in DMF at 25 °C. hThis sample had ηinh = 0.75 dL/g. Commercial Radel has ηinh = 0.45 dL/g. 2305


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features, for example, they take place without the production of any condensates. They are even greener if the reactions proceed well when catalyzed by enzymes6 or organic catalysts.141,193,194 The products are also then free of any metal residues. An example currently attracting interest is the ED-ROP of pentadecanolactone (20) catalyzed by a lipase,138 or organic catalysts.141,193,194 The polymer produced, polypentadecanolactone (131), has many material properties that are similar to polyethylene,138 and it has the potential to be used in biological situations. ED-ROPs are ideal for use in environmentally sensitive situations such as the preparation of coatings, films, or paints. Two examples were briefly discussed earlier. Thus, when a film of MCOs 13 plus a catalytic amount of stannoxane 12 is cast on a microscope slide then heated at 170 °C, it is transformed into a self-standing film of polymer 139.14 Similarly a cast film of MCO 55 is smoothly transformed by olefin metathesis at 40 °C into a self-standing film of polymer.26,74 5.2. Recycling

Clearly the combination of CDP plus ED-ROP could form the basis of a method for recycling certain condensation polymers. Polymers that have been successfully subjected to CDP were considered in sections 3.4 and 3.6. An attractive feature of CDP for polymer degradation is that, unlike, for example, the hydrolysis or methanolysis of polyesters, it does not add a pair of “end groups” to every monomer unit, end groups that in any subsequent polymerization need to be removed again, often with some difficulty, for example, by the use of high temperatures and/or reduced pressures. CDP immediately affords reusable monomers that incorporate no end groups. CDP does, however, have some significant problems that need to be addressed. For example, the reaction times are often long and may run into days. This suggests that better catalysts are required, preferably organic catalysts141,193,194 or enzymes.6 Further, when the CDP is carried out in the classical manner (see sections 3.4 and 3.6), high dilutions are needed, and this requires the use of relatively large volumes of solvent. This is not, therefore, a very green process. One possibility for the future is to carry out CDPs continuously and separate the MCOs from the polymer by means of size-selective membranes. It should be noted that normally there would be very few linears present other than polymer simply because normally there will not be enough potential end groups in the system. With this membrane-based approach, full CDP may not be necessary. Diluting the polymer such that at equilibrium there is, say, 20% of MCOs may well be quite sufficient.195 Clearly a membrane that is compatible with organic solvents, preferably at elevated temperatures, is required. In those cases where the MCOs are volatile, it may, as discussed in section 3.4, be possible to simply distill the lower MCOs directly from a reaction mixture. An example is the CDP of aliphatic polycarbonates.94 In this particular case, there is the added bonus that no catalyst is required. Polymer-supported catalysts may simplify the recovery of the catalyst. In appropriate cases it may be practical to use a supported catalyst in a flow system. Supported enzymes have been used to achieve CDPs in simple flow systems.119,121,196 Once the MCOs have been obtained, they can be used alone as the starting materials for ED-ROPs, or mixed in with the polymer and the mixture of MCOs and polymer used for EDROPs.197

withdrawing group. Thus, heating macrocycle 260 rapidly up to 480 °C in a DSC instrument does not result in ED-ROP, but heating 261 or 262 similarly does result in ED-ROPs; see Table 8, entries 7 and 8, respectively.191,192

5. SOME POTENTIAL APPLICATIONS OF ED-ROPS The main purpose of this article is to review polymer syntheses based on ED-ROPs and to consider some of the methods available to obtain the required starting MCOs. It is, however, appropriate to conclude this review by considering briefly some of the potential uses of ED-ROPs. The list of topics considered below is by no means exhaustive. 5.1. Greener Method of Polymer Synthesis

Methods that allow polymers to be synthesized in a greener way will always be of interest. ED-ROPs have several green 2306


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mer 164 with Mn = 146 000 and Mw = 273 500; see Table 7, entry 14.76

The key point here is that ED-ROP plus CDP has the potential to become a method for recycling many condensation polymers. Höcker has called the recycling of polymers by thermal degradation to give strained monomers, that can subsequently be repolymerized, “Thermodynamic Recycling”.198 Recycling using CDP to give strainless macrocycles plus ED-ROP could be described as “Ring−Chain Recycling” and would be more widely applicable.

5.5. Reactive Molding and Composites

One of the attractions of ED-ROPs is that little or no heat and no small molecules are evolved during the polymerization. This makes them particularly suitable, for example, for reaction injection molding (RIM). Thus, voids are not created in the products by the release of small volatile compounds. The fact that little or no heat is evolved makes control of the reaction more straightforward. A further advantage is that MCOs generally have much lower melt viscosities that the corresponding polymers. The lower melt viscosites have been quantitatively demonstrated for precursors of polymer 245 and similar MCOs,202 ester-containing MCOs 8, 29, and 116,195 and aromatic sulfone MCOs 68.203 Mixtures of MCOs and polymer also have reduced melt viscosities, and the presence of 10−20% of MCOs in a polymer can result in a significant reduction in viscosity.195,203 Because of their lower viscosities, molten MCOs can penetrate the smaller recesses of molds and mix with, and wet, for example, glass fibers more easily than the polymers can. Once in place, the MCOs are subjected to in situ ED-ROPs. A simple demonstration of how lower melt viscosities can be useful comes from the preparation of a composite of highperformance polymer 230 and a 150-μm mesh woven stainless steel cloth.57 When the cloth and polymer were heated together at 300 °C for 30 min, i.e., at approximately the Tm of the polymer (306 °C), the polymer and mesh did not interpenetrate significantly, but when the macrocyclic trimer 63: x = 3 and 1 wt % of cesium fluoride were heated similarly in the presence of the steel cloth, the hot MCO flowed rapidly into the mesh, so that when ED-ROP occurred the mesh became fully embedded in the polymer. How small a matrix can molten MCOs penetrate? To obtain an indication, attempts were made to mold microfibrils or tubes of high-performance polymers in the pores of 0.1-μm (nominal pore size) alumina filters, as studied for some other types of polymer.204 In these studies after the polymer is formed in the pores of the filter, the alumina is dissolved away with alkali to leave the polymeric material. The ED-ROP of MCOs 68 gave tubes of Radel (125) 300 nm in diameter, while with the monomer 68: x = 3 it gave 200−400 nm fibrils.178 Thus, it is certainly possible to obtain very detailed features in high-performance polymer moldings using EDROP. Recently experiments have been carried out to assess the possibility of using the reduced melt viscosities of blends of MCOs 238 and polymer 6 relative to the polymer itself to facilitate the processing of polymer 6. On heating at up to 430 °C, MCOs 238 underwent thermal ED-ROP to give polymer 239 as a blend with polymer 6.205 MCOs have been used for fabricating composites or nanocomposites. Thus, Monvisade’s group has prepared composites of hydroxyapatite and poly(ethylene glutarate),206 poly(ethylene terephthalate) (1),207 and poly(ethylene adipate) (140)208 by impregnating the hydroxyapatite with the appropriate MCOs and a catalyst and then heating to bring about ED-ROP. Nanocomposites of clays and polyesters 29,209,210 and 1,211 have been prepared by carrying out ED-ROPs of the appropriate MCOs in the presence of the clay. The products have been shown to have superior material properties.

5.3. High-Throughput Syntheses

There is growing interest in high-throughput (HTP) syntheses of polymers, often in order to prepare polymer libraries.24,156,199−201 For the polymerization of vinyl monomers,199 and for certain types of step-growth addition polymerization,200 standard synthesis procedures can be adapted relatively easily to give HTP synthesis procedures, but for syntheses of other types of condensation polymer HTP syntheses can be difficult. Thus, a conventional approach to certain condensation polymerizations may require carefully balanced proportions of the monomers and prolonged reaction times at elevated temperatures under vacuum with a need to stir viscous reaction mixtures. These conditions are obviously not easily adapted to large numbers of reactions on a small scale. One solution to this problem is to use ED-ROPs. An example is the HTP synthesis of a series of 9 ethylene and propylene terephthalate copolymers.156 A second example is the preparation of libraries of up to 13 copolyesters prepared on an approximately 100-mg scale by reacting together mixtures of ester-containing MCOs in small wells.24 Yields were excellent. In both studies analyses of several of the products, by 1H and/or 13C NMR spectroscopy, showed that the products had random sequences of the different types of repeat unit. As noted earlier, initially the product will be blocky and it will take time to become random.24 One variation on this ED-ROP approach to HTP synthesis is to carry out small-scale reactions in an automated DSC instrument. The polymers formed are then immediately available for thermal analysis. For examples, see refs 57, 58, and 81. 5.4. Synthesis of Polymers with High Molecular Weights

ED-ROPs using pure MCOs need a source of end groups to produce linear polymers; see Scheme 1. Often the end groups are derived from the catalyst. Providing an ED-ROP can be taken to completion, limiting the number of end groups in the system offers the opportunity to obtain products with unusually high molecular weights and a dispersity3 close to 2.0. A simple review of Tables 1−9 indicates that, although no serious attempts appear to have been made to maximize molecular weights, 18 ED-ROPs gave polymeric products with, for example, Mw’s > 100 000 and dispersities < 2.8. In seven of these examples, Mw was >200 000. The highest value was 280 000 for the synthesis of polymer 3.64,163 These values are far higher than those normally obtained for the synthesis of condensation polymers by a classical approach, such as the synthesis of poly(ethylene terephthalate) (1) outlined in Scheme 2,25 or the synthesis of olefin-containing polymers by acyclic diene metathesis (ADMET).74,76 ED-ROPs of macrocyclic olefins are particularly appropriate for obtaining high molecular weights because there is unlikely to be any adventitious olefins present and the catalysts, usually Grubbs’ catalysts 49 and 50, are highly active so that very little need be used, i.e., the catalyst will provide very few end groups. As an example ED-ROP of MCOs 56 gave poly2307


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Review

Biography

6. SUMMARY AND FUTURE WORK ED-ROPs are a new approach to the synthesis of a wide range of condensation polymers. This review discusses the synthesis of polyesters, polycarbonates, polyamides, polyacetals, olefincontaining polymers, and high-performance aromatic polymers by ED-ROPs. In most cases the syntheses are based on RCE; see Scheme 1. These equilibria lie heavily on the side of polymers at high concentrations, so carrying out ED-ROPs simply involves taking MCOs at high concentration and using reaction conditions that favor the establishment of the RCE. Dispersities are usually close to 2.0. Attractive features of ED-ROPs include the facts that little or no heat is evolved during polymerization and no small molecules are emitted. The reactions are easily carried out on a small scale, say, 100 mg. By restricting the availability of end groups, unusually high molecular weights can be obtained. In many cases, without specifically seeking to obtain unusually high molecular weights, Mw’s of up to 250 000 with dispersities < 2.8 have been obtained. ED-ROPs are beginning to find applications as a green method of polymerization, for high-throughput syntheses, and for polymer processing, such as reactive molding and in the synthesis of composites. The relatively low melt viscosity of the MCOs facilitates some methods of processing. Because the required MCOs can be obtained by reversing the polymer synthesis, i.e., by cyclodepolymerization (CDP) (see Scheme 1), there is the potential to recycle certain polymers by the combination of CDP plus ED-ROP, i.e., “Ring−Chain Recycling”. Future work is needed to develop efficient syntheses of a wider range of MCOs. These will almost certainly be based on pseudohigh-dilution methods. Currently there are good methods for the synthesis of the MCOs required for the synthesis of polyesters and aromatic polycarbonates. Pseudohigh-dilution syntheses of MCOs for the synthesis of polyamides and various high-performance aromatic polymers syntheses are reasonably efficient. Better catalysts, preferably metal-free catalysts, are needed to reduce reaction times for both ED-ROPs and CDPs and to make them greener. When CDPs are more rapid and the “highdilution problem” (see section 5.3) is solved, the use of EDROP plus CDP may become a viable commercial method for recycling certain polymers. Certain ED-ROPs and CDPs take place without the need for catalysts, and such reactions are worthy of further study. The currently known reactions of this type involve aliphatic carbonates and various sulfur-containing molecules like thioesters and aromatic para-thioketones. Only a few such groups per polymer chain are probably needed. Finally, the principles discussed in this review apply equally well to other inorganic or partially inorganic systems such as sulfur and siloxanes.

The author obtained his Ph.D. at the University of Manchester in 1963 under the supervision of Professor Arthur J. Birch and Rod W. Rickards. He then carried out research at the University of Oxford with Professor Sir Ewart R. H. Jones and at Syntex (Palo Alto) before joining the staff of Lancaster University in 1966, becoming a professor in 1985. He moved to the Chair of Polymer Chemistry in the Department of Chemistry at Manchester University in 1989. Recognizing that many of Nature’s key molecules are reactive macromolecules, he chose to carry out research at the polymer chemistry−organic chemistry interface. He pioneered the study of polymer-supported organic chemistry in the 1970s and 1980s before it became very popular. In this period he reported early examples of organic reactions using supported reagents, catalysts (especially chiral catalysts), substrates, protecting groups, and catch-and-release systems. He also reported early examples of simple flow systems using supported reagents or supported catalysts (other than the traditional supported strong acids). He coedited two books on polymersupported organic chemistry in the 1980s. In recent years he has focused more on using organic methods to achieve polymer synthesis, especially on entropically driven ring-opening polymerizations (EDROPs) of strainless macrocycles. He also has many papers on the subject of polymeric Langmuir−Blodgett films and their applications, especially in optoelectronics. He has more than 300 publications and has been awarded several research prizes and awards.

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +44 (0)161 275 4710. Fax: +44 (0)1524 793 252. Notes

The authors declare no competing financial interest. 2308


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